1. Field of the Invention
This invention relates to methods and materials for the dehydrogenation of alkanes, particularly the conversion of ethane to ethylene, and more particularly, to mixed oxide catalysts for oxidative dehydrogenation of ethane.
2. Discussion
Ethylene can be produced by thermal cracking of hydrocarbons, and by nonoxidative dehydrogenation or oxidative dehydrogenation of ethane (ODHE). The latter process is attractive for many reasons. For example, compared to thermal cracking, high ethane conversion can be achieved at relatively low temperatures (about 400xc2x0 C. or below). Unlike thermal cracking, catalytic ODHE is exothermic, requiring no additional heat to sustain reaction. Furthermore, in contrast to catalytic dehydrogenation, catalyst deactivation by coke formation should be minimal in ODHE because of the presence of oxygen in the reactor feed. Other alkanes can similarly be oxidatively dehydrogenated.
In the late seventies, Thorsteinson and coworkers first disclosed useful low temperature ODHE catalysts comprised of mixed oxides containing molybdenum, vanadium, and a third transition metal. E. M Thorsteinson et al., xe2x80x9cThe Oxidative Dehydrogenation of Ethane over Catalyst Containing Mixed oxide of Molybdenum and Vanadium,xe2x80x9d 52 J. Catalysis 116-32 (1978). Later studies examined families of alumina-supported vanadium-containing oxide catalysts, MV and MVSb, where M is Ni, Co, Bi, and Sn. R. Juarez Lopez et al., xe2x80x9cOxidative Dehydrogenation of Ethane on Supported Vanadium-Containing Oxides,xe2x80x9d 124 Applied Catalysis A: General 281-96 (1995). More recently, Schuurman and coworkers describe unsupported iron, cobalt and nickel oxide catalysts that are active in ODHE. Y. Schuurman et al., xe2x80x9cLow Temperature Oxidative Dehydrogenation of Ethane over Catalysts Based on Group VIII Metals,xe2x80x9d 163 Applied Catalysis A: General 227-35 (1997). Although the mixed oxide catalysts reported by Thorsteinson, Schuurman and others might be useful discoveries, they represent a small fraction of potentially active inorganic oxide mixtures.
Industrial interest has stimulated investigations into new catalysts and methods for improved performance (e.g., conversion and selectivity) for the oxidative dehydrogenation of alkanes. There is a need for new dehydrogenation catalysts and methods.
This invention discloses catalysts and methods for the oxidative dehydrogenation of alkanes that have from 2 to 4 carbon atoms and particularly ethane to ethylene. These catalysts primarily include nickel oxide and catalyze oxidative dehydrogenation with conversions of greater than 5% and with selectivity of greater than 70%. An object of the present invention is to provide a process whereby a C2-C4 alkane can be oxidatively dehydrogenated to one or more olefins with relatively high levels of conversion and selectivity. A further object of this invention is to provide a catalyst that selectively catalyzes the reaction of a C2-C4 alkane with oxygen to produce one or more corresponding C2-C4 olefins with relatively high levels of conversion and selectivity, meaning preferably without the concurrent production of significant amounts of by-products, such as carbon monoxide or carbon dioxide.
In general, the catalysts of this invention have as a required component nickel oxide and it is an object of this invention to provide a catalyst for the oxidative dehydrogenation of an alkane into one or more olefins having nickel oxide (NiO). The nickel oxide is combined with other metal oxides, dopants, carriers, binders and/or fillers into a catalyst that is contacted with a gas mixture. The gas mixture comprises at least the alkane and oxygen, but may also include diluents (such as argon, nitrogen, etc.) or other components (such as water or carbon dioxide). Optionally, the gas mixture that contacts the catalyst may also include one or more of the olefin products for an oxidative dehydrogenation process that converts a gas mixture having one ratio of alkane to alkene to a gas mixture having a different ratio of alkane to alkene. The catalysts of this invention include a material or composition of matter having the empirical formula:
NixAjBkClOixe2x80x83xe2x80x83I
wherein Ni is nickel and x is in the range of about 0.1-0.96;
A is selected from the group consisting of Nb, Ta, Co and combinations thereof and j is in the range of from about 0-0.8;
B is an element selected from the group consisting of alkali metals, alkaline earths, or lanthanides and combinations thereof, including Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Mn, La, Ce, Pr, Nd, Sm and combinations thereof and k is in the range of from 0-0.5;
C is an element selected from the group consisting of Sn, Al, Fe, Si, B, Sb, Tl, In, Ge, Cr, Pb and combinations thereof and 1 is in the range of from 0-0.5;
i is a number that satisfies the valence requirements of the other elements present; and
the sum of j, k and 1 is at least 0.04.